(1) Field of the Invention
The present invention relates to a phased array antenna system with adjustable electrical tilt. It is suitable for use in many areas of telecommunications but finds particular application in cellular mobile radio networks, commonly referred to as mobile telephone networks. More specifically, but without limitation, the antenna system of the invention may be used with second generation (2G) mobile telephone networks such as the GSM system, and third generation (3G) mobile telephone networks such as the Universal Mobile Telephone System (UMTS).
(2) Description of the Art
Operators of cellular mobile radio networks generally employ their own base-stations, each of which has at least one antenna. In a cellular mobile radio network, the antennas are a primary factor in defining a coverage area in which communication to the base station can take place. The coverage area is generally divided into a number of overlapping cells, each associated with a respective antenna and base station. The cells are also generally divided into sectors to increase the communications coverage.
The antenna of each sector is connected to a base station for radio communication with all of the mobile radios in that sector. Base stations are interconnected by other means of communication, usually point-to-point radio links or fixed land-lines, allowing mobile radios throughout the cell coverage area to communicate with each other as well as with the public telephone network outside the cellular mobile radio network.
Cellular mobile radio networks which use phased array antennas are known: such an antenna comprises an array (usually eight or more) individual antenna elements such as dipoles or patches. The antenna has a radiation pattern consisting of a main lobe and sidelobes. The centre of the main lobe is the antenna's direction of maximum sensitivity, i.e. the direction of its main radiation beam. It is a well known property of a phased array antenna that if signals received by antenna elements are delayed by a delay which varies linearly with distance from an edge of the array, then the antenna main radiation beam is steered towards the direction of increasing delay. The angle between main radiation beam centres corresponding to zero and non-zero variation in delay, i.e. the angle of steer, depends on the rate of change of delay with distance across the array.
Delay may be implemented equivalently by changing signal phase, hence the expression phased array. The main beam of the antenna pattern can therefore be altered by adjusting the phase relationship between signals fed to different antenna elements. This allows the beam to be steered to modify the coverage area of the antenna.
Operators of phased array antennas in cellular mobile radio networks have a requirement to adjust their antennas' vertical radiation pattern, i.e. the pattern's cross-section in the vertical plane. This is necessary to alter the vertical angle of the antenna's main beam, also known as the “tilt”, in order to adjust the coverage area of the antenna. Such adjustment may be required, for example, to compensate for change in cellular network structure or number of base stations or antennas. Adjustment of antenna angle of tilt is known both mechanically and electrically, and both individually or in combination.
Antenna angle of tilt may be adjusted mechanically by moving antenna elements or their housing (radome): it is referred to as adjusting the angle of “mechanical tilt”. As described earlier, antenna angle of tilt may be adjusted electrically by changing time delay or phase of signals fed to or received from each antenna array element (or group of elements) without physical movement: this is referred to as adjusting the angle of “electrical tilt”. When used in a cellular mobile radio network, a phased array antenna's vertical radiation pattern (VRP) has a number of significant requirements:                1. high main lobe (or boresight) gain;        2. a first upper side lobe level sufficiently low to avoid interference to mobiles using a base station in a different cell or network;        3. a first lower side lobe level sufficiently high to allow communications in the immediate vicinity of the antenna.        
These requirements are mutually conflicting: for example, increasing the boresight gain may increase the level of the side lobes. A first upper side lobe level, relative to the boresight level, of −18 dB has been found to provide a convenient compromise in overall system performance.
The effect of adjusting either the angle of mechanical tilt or the angle of electrical tilt is to reposition the boresight so that it points either above or below the horizontal plane, which changes the coverage area of the antenna.
It is desirable to be able to vary both the mechanical tilt and the electrical tilt of an antenna of a cellular radio base station: this allows maximum flexibility in optimisation of cell or sector coverage, since these forms of tilt have different effects on antenna ground coverage and also on other antennas in the station's immediate vicinity. Moreover, operational efficiency is improved if the angle of electrical tilt can be adjusted remotely from the antenna assembly. Whereas an antenna's angle of mechanical tilt may be adjusted by repositioning its radome, changing its angle of electrical tilt requires additional electronic circuitry which increases antenna cost and complexity. Moreover, if a single antenna is shared between a number of operators, it is preferable to provide an individual angle of electrical tilt for each operator.
The need for an individual angle of electrical tilt from a shared antenna has hitherto not been met and has resulted in compromises in system performance. Further reductions in system performance may also occur if the gain decreases as a consequence of the technique adopted to change the angle of electrical tilt.
R. C. Johnson, Antenna Engineers Handbook, 3rd Ed 1993, McGraw Hill, ISBN 0-07-032381-X, Ch 20, Figure 20-2 discloses a method for locally or remotely adjusting the angle of electrical tilt of a phased array antenna. In this method, a radio frequency (RF) transmitter carrier signal is fed to the antenna and distributed to the antenna's radiating elements. Each antenna element has a variable phase shifter associated with it so that signal phase can be adjusted as a function of distance across the antenna to vary the antenna's angle of electrical tilt. The distribution of power when not tilted is proportioned so as to set the side lobe level and boresight gain. Optimum control of the angle of tilt is obtained when the phase front is controlled for all angles of tilt so that the side lobe level is not increased over the tilt range. The angle of electrical tilt can be adjusted remotely, if required, by using a servo-mechanism to control the position of the phase shifters.
This prior art method antenna has a number of disadvantages. A variable phase shifter is required for every antenna element. The cost of the antenna is high due to the number of such phase shifters required. Cost may be reduced by using a single common delay device or phase shifter for a group of antenna elements instead of per element, but this increases the side lobe level. See for example published International Patent Application No. WO 03/036756 A2 and Japanese Patent Application No. JP20011211025 A.
Mechanical coupling of delay devices may be used to adjust delays, but it is difficult to do this correctly; moreover, mechanical links and gears result in non-optimum distribution of delays. The upper side lobe level increases when the antenna is tilted downwards, thus causing a potential source of interference to mobiles using other base stations. If the antenna is shared by a number of operators, the operators then have a common angle of electrical tilt instead of different angles which is preferable. Finally, if the antenna is used in a communications system having up-link and down-link at different frequencies (frequency division duplex system), the angle of electrical tilt in transmit mode is different from that in receive mode because of frequency dependence of properties of signal processing components.
International Patent Application Nos. PCT/GB2002/004166 and PCT/GB2002/004930 describe locally or remotely adjusting an antenna's angle of electrical tilt by means of a difference in phase between a pair of signal feeds connected to the antenna.